It is known from U.S. Pat. Nos. 3,343,930 and 3,393,089 to coat metallic surfaces with an alloy of zinc and aluminum for corrosion protection. The method according to which these alloys are coated onto the metallic surfaces comprises dipping the metal substrate into a molten bath of the respective alloys. The advantages of zinc-aluminum coatings compared to pure aluminum coatings or alloys of aluminum with e.g. refractory metals such as titanium, chromium or molybdenum is that zinc provides a higher galvanic protection to e.g. a steel substrate. Thus the steel surfaces which are coated with a zinc-aluminum alloy provide a better corrosion protection than pure aluminum or alloys of aluminum with refractory metals.
Even though the above two U.S. patents already disclose the use of zinc-aluminum alloys for improved galvanic corrosion protection, the proposed hot dipping process does not provide optimum coatings in respect of uniformity, optimal thickness and formability. In addition the high temperature process leads to the formation of brittle intermetallic phases at the coating-substrate interface and requires the addition of silicon to reduce this tendency and to improve the coating adhesion.
The U.S. Pat. No. 4,287,009 discloses an Al/Zn coating which is applied to the substrate by a hot dipping process, whereby the exact temperature during the dipping process and the cooling rate are controlled as to improve the structure of the coating. Compared to uncontrolled dipping processes it is claimed to reduce the grain size and therewith improve the performance of the coating. However, it is evident from the phase diagram of the Al/Zn system that no single phase solidification is obtainable since the area in the phase diagram which is designated as ".alpha.+L" has to be crossed during the cooling period, which needs a certain amount of time, during which the composition of the liquid phase changes gradually because of the solidification of selected components. Therefore the solidification leads first to Al rich zones and then to Zn rich zones, thus creating at least a two phase coating. The grain sizes are coarse, being of the order of up to 50 microns. The only possibility to obtain a single phase solidification in this system would be to pass exactly through the eutectic point which is, however, only possible for a mixture of approximately 5% Al and 95% Zn. The shock cooling after complete solidification may decrease the grain size thereafter by interdiffusion, but does not lead to a real submicroscopic distribution, which is believed to yield the best performance for corrosion protection, since relatively large Al-rich zones tend to become passivated by the formation of a protective oxide layer. Consequently the zinc is subject to corrosion leaving a porous passivated Al structure which is less galvanically active and offers poorer corrosion protection to the substrate.
From U.S. Pat. No. 3,775,260 it is known to electroplate aluminum or e.g. an alloy of aluminum and zinc or cadmium onto a metallic substrate whereby the electrodeposition is carried out in a hydrous organic solvent comprising aluminum bromide. The second metal component of the desired alloy is introduced as a soluble alloy anode which gradually dissolves as the respective metal component is plated onto the cathode. However, there are several disadvantages in the use of the hydrous electrolyte. The reduction of protons will lead to a loss of cathodic current efficiency. In addition the proton reduction can lead to hydrogen embrittlement problems when high strength steels are coated. Moreover, this U.S. patent does not refer to any use of the coatings produced with the disclosed metals and no specific advantages of the obtained coatings in respect to other coating methods are discussed.
U.S. Pat. No. 2,170,375 discloses a method for electroplating aluminum or an alloy thereof with zinc, copper or cadmium onto a substrate of a dissimmilar metal in a non-aqueous bath comprising the reaction product of benzene or derivatives of benzene and alkyl or hydrogen halides. However, alkyl halides lead to the formation of alkylhalogenoaluminate species which are not cathodically stable. Consequently these species can be reduced during electrolysis leading to a decrease of current efficiency and chemical decomposition of the solvent system. Again no specific use of this method and no product which may be manufactured by this method is disclosed and no specific advantages of this plating method over other known coating procedures are discussed.
H. G. Read et al (ELECTROCHEMICAL TECHNOLOGY, Vol 4, No. 11-12, Nov/Dec. 1966) report electrolytic codeposition of aluminum and manganese in fused-salt baths to form bright coatings. Details are discussed relating to the optical aspects of these coatings depending on the concentration of manganese in the electrolyte and in the deposit. The coatings produced from this relatively high temperature system are reported by the authors to be crystalline.
There have been no reports of the direct electrochemical preparation of amorphous aluminum alloys. In addition it has not proved possible to prepare amorphous aluminum manganese alloys by the conventionally used rapid quenching technique.